Low pressure mercury vapor discharge lamp

ABSTRACT

An improved low pressure mercury vapor discharge lamp having an elongated vitreous envelope with electrodes sealed into its ends. At least one of said electrodes is connected to an outwardly flared stem which is sealed to one end of the envelope. The stem is partially coated with an amalgam-forming metal which becomes the main amalgam during lamp operation. The stem is also coated with a ceramic between the amalgam-forming metal and the electrode to prevent migration of the amalgam during operation.

United States Patent 1191 Herrmann et al.

[ Dec. 31, 1974 LOW PRESSURE MERCURY VAPOR DISCHARGE LAMP [75] Inventors: Rolf Herrmann, Sauerlach; Giinther Sohring, Munich, both of Germany [73] Assignee: Patent-Treuhand-Gesellschaft fur electrische Gluhlampen mbH,

Munich, Germany [22] Filed: Jan. 16, 1974 [211 App]. No.: 433,773

[30] Foreign Application Priority Data Feb. 6, 1973 Germany 2305753 [52] US. Cl 313/178, 313/176 [51] Int. Cl. HOlj 61/28 [58] Field of Search 313/178 [56] References Cited UNITED STATES PATENTS 3,548,24l 12/1970 Rasch et al. 313/178 Primary Examiner-Rudolph V. Rolinec Assistant Examiner-Darwin R. Hostetter Attorney, Agent, or Firm-Flynn & Frishauf [57] ABSTRACT 10 Claims, 2 Drawing Figures PATENTEDBEB3 1 mu- 858,075

SHEET 10F 2 FIG. 1

FEATENTED UEE3 1 I974 SHEET 20F 2 FIG. 2

LOW PRESSURE MERCURY VAPOR DISCHARGE LAMP This invention relates to low pressure mercury vapor discharge lamps and more particularly to fluorescent lamps containing an amalgam. In these lamps the amalgam characteristics are such that the mercury vapor pressure is lower than that of free mercury at the same temperature.

It is well known that the location of the amalgamforming metal and/or of the amalgam within the lamp effects the function of the amalgam. Similarly, the composition of the amalgam, both in connection with the selection of the amalgam-forming metal and the relative quantities of the amalgam-forming metal and the mercury, effect the function of the amalgam. Similarly, the selection of the amalgam-forming metal or metals and the amounts thereof are influenced by their respective locations. The temperature prevaling during operation of the lamp at the location of the amalgam-forming metal and/or the amalgam is also of decisve importance in connection with the positioning thereof and the determination of the components thereof and the relative proportion of each. The lamp may also contain several amalgams which have different functions. The main amalgam, as is well known, determines the mercury vapor pressure during operation of the lamp. One or more other amalgams may be utilized to obtain a reduction of the run-up period of the lamp.

The present invention provides improvements in prior art lamps such as those disclosed in US. Pat. No. 3,548,241 and US. Pat. No. 3,629,641, which are hereby incorporated by reference. These patents disclose the provision of a coating of the amalgamforming metal on the flared portion of the stem. The coating is preferably in the shape of an annulus (ring) covering at least a portion of the flared section of the stem. Thus, it may be wholly positioned on the flared section or it may be partially positioned on the flared section and the remainder on a connecting substantially more cylindrical section. The amalgam-forming metal is preferably deposited by a spray coating process.

It has now been discovered that when such lamps are operated for extended periods such as one thousand hours, under certain conditions, the amalgam may migrate. An explanation of the cause of such migration of amalgam is not definitive. The temperature gradient along the lamp stem which increases in the direction from the flare toward the electrode is an important factor. Other factors include the specific amalgamforming metal and the physical state of such amalgam, for example, the amount of aggregation; and also the wettability characteristics of the amalgam-forming metal and/or the amalgam for the glass stern. It has been found that when using indium which is the preferred amalgamforming metal and which has a substantial ability to wet glass, the amount of migration increases as the temperature gradient is increased.

Since the temperature in the area of the tube immediately proximate the amalgam affects the operating characteristics of the lamp and particularly the optimum lamp characteristics, and since the temperature immediately proximate the amalgam varies with the positioning of the amalgam in the tube, it follows that migration of the amalgam-forming metal and/or of the amalgam results in a change of the optimum lamp characteristics during operation of the lamp. This is an undesirable consequence of the migration. The present invention provides improved lamps which overcome this disadvantage of the prior art lamps.

[t is an object of the invention to provide mercury vapor discharge lamps having uniform operating characteristics over an extended service life.

SUBJECT MATTER OF THE INVENTION:

The present invention provides a low pressure mercury vapor discharge lamp having improved operating characteristics. The lamp comprises an elongated vitreous envelope having electrodes sealed into its ends. At least one of said electrodes is connected to an outwardly flared stem which is sealed to one end of said vitreous envelope. The said flared stem is partially coated with an amalgam-forming metal which becomes the main amalgam during operation of the lamp. Migration of the amalgam during operation is prevented by positioning a ceramic coating on the flared stem between the amalgam-forming metal and the electrode.

The ceramic material is applied as a coating on the stem. The ceramic material should not react with the amalgamforming metal nor with the subsequent amalgam. It should also not react with the material of the glass stem nor with the gaseous components of the lamp. The ceramic material must also not give off materials (usually vapor) during the manufacture of the lamp nor during the service life of the lamp. Such materials which should not be given off include decomposition products of the ceramic material and/or the dissipation and/or vaporization of adsorbed gases or moisture in the ceramic material.

The preferred ceramic material is ZrO SiO coatings in the form of powdered quartz are also useful as the ceramic coatings. The ceramic coating may be applied in the form of a suspension of the powdered ceramic in a liquid which is brushed or sprayed on the glass stem.

The preferred amalgam-forming materials are indium or an indium alloy. lndium is particularly preferred. The amalgam-forming material is preferably spraydeposited on a flared portion of the glass stem. the amalgam-forming metal is preferably in the form of an annular coating and thus forms a ring around the stem. The annular coating may be wholly on the flared portion of the glass stem or partially on the flared portion of the stem and partially on the adjoining more substantially cylindrical portion of the stem. The ceramic coating is preferably immediately adjacent said amalgamforming coating and is preferably also an annular coating. The ceramic coating is positioned on the glass stem in the direction of the electrode, i.e., in the direction away from the end of the mercury discharge lamp.

An embodiment of the lamp according to the invention is illustrated in the accompanying figures, wherein:

FIG. 1 is a side view, partially in section, of the electrode stem, and

FIG. 2 is a partial sectional view of the fluorescent lamp containing the amalgam.

The electrode stem 1 comprpises a flared tube portion 2 and stem press 3. Lead wires 4 and S are passed through the stem press 3 and flare tube 2 to the outer plug connections. The electrode coil 6 is surrounded by an annular metal cap 7 composed ofiron foil. The electrode coil 6 is attached to the inner ends of lead wires 4 and 5. The annular metal cap 7 is supported by wire 8. A strip 9 of amalgam-forming alloy is positioned on said annular cap 7. The amalgam formed on strip 9 is adapted to facilitate run-up of the lamp.

The main amalgam of the lamp is formed during operation of the lamp on an annular strip 10 of amalgamforming metal. Preferred lampscontain a strip about 3 mm wide of indium. This strip of indium is applied by spray-coating on flared tube 2. An annular coating 11 of ceramic material is positioned immediately adjacent said annular coating 10 of amalgam-forming material. The ceramic coating ring is also of a width of about 3 In the preferred lamp which is illustrated in FIG. 2, the electrode stem 1 is illustrated sealed to the lamp en velope 12. The lamp envelope 12 is provided with an inner phosphor coating 13.

Preferred lamps having the construction illustrated in FIGS. 1 and 2 may be prepared with the annulus of 3 mm width formed of a spray-deposited coating of the preferred amalgam-forming metal indium. The total amount of indium is about 55 mg. In operation of the lamp, this becomes the main amalgam. In the illustrated lamp, the ratio of indium to mercury in the amalgam formed after introduction of mercury into the lamp, is 5.511. In the illustrated lamp, and using the preferred ZrO as the ceramic material, there is an amount of between about 3 and 5 mg. to form the illustrated 3 mm wide annular coating.

The ceramic coating alters the ability of the amalgam and/or amalgam-forming metal to wet the glass stem. It also effects accumulation of heat in the glass stem as a result of its insulating characteristics. The foregoing is offered as a possible explanation for the advantageous effects flowing from the use of the ceramic material.

Lamps produced in accordance with the drawings and the example hereinbefore have operated in excess of one thousand hours without migration of the amalgam. These lamps have operated with optimum operating characteristics over this lengthy service and without decrease in light intensity resulting from changes in operating characteristics. The ceramic coating is preferably very adherent to the stem and the annulus is preferably sharply delineated.

What is claimed is:

1. In a low-pressure mercury vapor discharge lamp comprising an elongated vitreous envelope having electrodes sealed into its ends, at least one of said elec trodes being connected to an outwardly flared stern sealed to one end of said envelope and said stem being partially coated with an amalgam-forming metal which becomes the main amalgam during lamp operation, the improvement comprising positioning a ceramic coating on said stem between said amalgam-forming metal and said electrode.

2. The lamp of claim 1 wherein said amalgamforming meal is in the form of an annulus covering at least part of the flared portion of said stem, and said ceramic coating is an adjacent annular coating; and wherein said amalgam-forming annular coating is a spray-deposited coating.

3. The lamp of claim 2 wherein said amalgamforming metal coating is indium.

4. The lamp of claim 3 wherein said ceramic material is ZrO 5. The lamp ofclaim 3 wherein said ceramic material is SiO 6. The lamp ofclaim 1 wherein said ceramic material is SiO 7. The lamp of claim 1 wherein said ceramic material is ZrO 8. The lamp of claim 1 wherein said amalgamforming metatl is indium.

9. The lamp of claim 2 wherein said ceramic material is ZrO 10. The lamp of claim 2 wherein said ceramic material is SiO 

1. IN A LOW-PRESSURE MERCURY VAPOR DISCHARGE LAMP COMPRISING AN ELONGATED VITREOUS ENVELOPE HAVING ELECTRODES SEALED INTO ITS ENDS, AT LEAST ONE OF SAID ELECTRODES BEING CONNECTED TO AN OUTWARDLY FLARED STEM SEALED TO ONE END OF SAID ENVELOPED AND SAID STEM BEING PARTIALLY COATED WITH AN AMALGAM-FOAMING METAL WHICH BECOMES THE MAIN AMALGAM DURING LAMP OPERATION, THE IMPROVEMENT POSITIONING A CERAMIC COATING ON SAID STEM BETWEEN SAID AMALGAM-FORMING METAL AND SAID ELECTRODE.
 2. The lamp of claim 1 wherein said amalgam-forming meal is in the form of an annulus covering at least part of the flared portion of said stem, and said ceramic coating is an adjacent annular coating; and wherein said amalgam-forming annular coating is a spray-deposited coating.
 3. The lamp of claim 2 wherein said amalgam-forming metal coating is indium.
 4. The lamp of claim 3 wherein said ceramic material is ZrO2.
 5. The lamp of claim 3 wherein said ceramic material is SiO2.
 6. The lamp of claim 1 wherein said ceramic material is SiO2.
 7. The lamp of claim 1 wherein said ceramic material is ZrO2.
 8. The lamp of claim 1 wherein said amalgam-forming metatl is indium.
 9. The lamp of claim 2 wherein said ceramic material is ZrO2.
 10. The lamp of claim 2 wherein said ceramic material is SiO2. 